In recent years, in order to reduce costs or to effectively use existing equipment without establishing new communication equipment, a “power line modem” that can perform communications using an existing power line (also called an indoor wire) is attracting attention. This power line modem networks electric products which are arranged in or outside houses or in buildings, factories, shops, and the like, and which are connected through a power line, and thereby various processes such as control of these products, data communications, and the like can be performed.
As wire-based digital communication except for the digital communication obtained by networking electric products, i.e., as a network communication using a plurality of multiple-connected power line modems provided outside houses, internet communication using personal computers is known.
However, in the communication through the power line, transmission qualities between nodes connected to the power line are considerably different from each other depending on environments of electric products connected to the power line, although the nodes are connected to the same power line. The difference between the transmission qualities makes a large delay difference in data transmission even though data having equal data lengths is transmitted. Therefore, the respective nodes cannot perform communication in an optimum state, disadvantageously.
In particular, in order to realize high-speed communication, various communication parameters such as using frequency bands, transmission rates, and error correction methods of nodes on a power line must be optimally set.
For example, FIG. 13 is a diagram which shows the schematic configuration of the communication system using such a power line. In FIG. 13, this communication system has a plurality of nodes N1 to N3 connected to a power line W. Since a transmission quality in the power line W between the nodes N1 and N2 is deteriorated by a noise source NS such as an air conditioner, communication process sections 11 to 13 of the nodes N1 to N3 perform negotiations between the nodes N1 to N3 to optimally set communication parameters D11 to D13 such as the using frequency bands, transmission rates (baud rates D21 to D23), and error correction methods explained above. A communication control sections 21 communicates with another node by using the communication parameters D11 to D13 which are optimally set.
Since the transmission quality between the nodes N1 and N2 is deteriorated by the noise source NS, low-speed communication, i.e., a low baud rate is set. Since the transmission quality between the nodes N2 and N3 is not deteriorated, high-speed communication, i.e., a high baud rate is set.
Therefore, when packets having equal data lengths are transmitted, the power line between the nodes N1 and N2 and the power line between the nodes N2 and N3 are different from each other in transmission time. More specifically, as shown in FIG. 14, when a packet PK10 having a data length LE10 are transmitted from the node N1 to the node N2, a time t12 is required. However, the packet PK10 is transmitted from the node N2 to the node N3, the packet PK10 is transmitted in a time t11 which is shorter than the time t12.
As a result, data transmission between the nodes N1 and N2 occupies the power line W for a time longer than a time for which data transmission between the nodes N2 and N3 occupies the power line W. Low-speed inter-node communication having poor transmission quality hinders high-speed inter-node communication. In an extreme state, the high-speed inter-node communication may be prevented from being performed.
Since the power line W is occupied by the low-speed inter-node communication, data communication such as voice communication or motion image communication having time-critical properties is adversely affected. Therefore, proper communication corresponding to each type of data cannot be performed.
Furthermore, since transmission quality is deteriorated in low-speed inter-node communication, repeat request may occur frequently. In addition to the low-speed communication, quantity of data transmission also increases, and the power line W may be congested.
Therefore, it is an object of this invention to provide a communication system and a communication method for efficiently performing communications between nodes by using a transmission medium such as a power line that the communication qualities change depending on connection positions or on connection time zones of a plurality of nodes connected on the same transmission medium, and particularly for reliably securing communications having time-critical properties.